Chemical Reactions
Table of Contents
5. Discuss the difference between Physical Change and Chemical Change
5.1 Physical Change and Chemical Change
There are only 2 types of changes that we experience in life - physical change and chemical changes. Even your body and your psychology will change via physical and chemical means
If you take your computer and smash it with a hammer - then you can glue it back together (it probably won't work) this is an example of a Physical Change. The computer has changed Physically from a whole computer into pieces of computer. Likewise, heating up your computer till it starts to melt (but doesn't burn) is a physical change. Changing states, from solid to liquid, is a physical change as nothing new is made. Solid plastic and liquid plastic are both plastic.
If you take your computer and set fire to it, then it cannot be glued back together. The plastic turns black, the screen changes, there's lots of black smoke. This blackness are the carbons that are left over after oxygen from the air reacts with the carbons and hydrogens in the plastic forming gases in the smoke, such as Carbon Dioxide. The Carbon atoms were always there, but by combining with oxygen they have form a new compound, Carbon Dioxide. This is a Chemical change because you are getting new Compounds
Physical Change = changed Physically
Chemical Change = new Compounds
5.2 Chemical Change = Chemical Reactions
Most atoms don't like to be alone. In fact, of the 92 naturally occurring elements, only 6 can be alone.
The other 86 elements atoms will bond
Atoms can bond to atoms of the same element making molecules
Or atoms can bond to atoms of different elements, making Compound molecules (just called Compounds)
Helium, He, doesn't bond with anything. It is one of the 6 that are too good to bond with any others. These 6 are called Noble Gases.
Oxygen can bond with itself, making molecular Oxygen, O2
Oxygen can also bind with atoms of other elements. For example, Oxygen can bond to Carbon to make Carbon Monoxide CO. It can also make Carbon Dioxide, CO2
Iron, Fe, sits next to other Fe atoms. Iron can bind to Oxygen. This can be a very slow reaction. When Iron finally binds with Oxygen, we get Iron Oxide, FeO. This is Rust. This slowness is why it takes a long time for an Iron Nail to rust.
5.2 Chemical Reactions
For Atoms to React, they must crash into each other
If they crash into each other slowly, then they just bounce off each other and nothing happens
If they crash into each other fast, then they can stick together
Temperature causes atoms to move faster
This is why fire and heat is often used.
Some atoms that aren't crashing into each other fast enough to stick together at room temperature, do when you add fire
For example, Hydrogen and Oxygen. If you take a Hydrogen balloon and pop it with a needle, then you hear the balloon pop and nothing happens. This is because the Hydrogen and Oxygen collide into each other after the balloon is popped and just bounce off each other
If you pop a Hydrogen balloon with a match, then the oxygen around the match is moving very fast due to the heat. This fast moving oxygen smashes into the Hydrogen, forming H2O. This collision releases heat, this heat causes all of the surrounding Hydrogens and Oxygens to speed up insanely, they all smash into each other, sticking together, releasing more heat and light, causing more to speed up. This huge chain reaction of speed, collisions, heat, light, speed, collisions, heat light - is seen as a hydrogen balloon explosion.
Knowing that temperature effects the speed of a reaction, we can slow reactions down by doing the reaction somewhere cold. And, we can speed up the reaction by doing it somewhere hot
The atoms crashing into each other are called Reactants. When the reactants crash and stick together, they make a new compound, this compound is called a Product
The signs that a Chemical Reaction is occurring are as follows:
Temperature - gets hotter or colder
Gas - smelt or seen
light
Precipitate (solid stuff in a liquid)
permanent colour changes
5.3 Collision Theory
Atoms collide, if they are going fast enough they will stick together
When the atoms collide, and they are going fast enough, they might still not stick together
To stick together, they must be the compounds must be the right way round for the sticking to happen - kind of like the Velcro Ball game - where you catch a tennis ball with a velcro pad - it the pad is the wrong way round, then it doesn't matter how fast the tennis ball is going, it just won't stick
Another analogy is the Catchoo game - watch the clip and think about how it could be an analogy
5.4 Speeding up and slowing down Chemical Reactions
To increase the rate of chemical reactions, you can:
Increase temperature
The hotter it is, the more energy the atoms have so the faster they move
Increase the number of reactants
The greater the concentration, the more reactants, more collisions that can happen
Increase the surface area
This can be done by making the particles smaller - for instance a powder will react faster than chunks
Add a catalyst
This is something that will hold the compounds in the correct orientation for the collision to be successful. Catalysts help in the reaction but are not used in the reaction
5.5 Six Brilliant Lectures on Chemical Reactions from The Royal Institution
6 Hydrogen, Oxygen and Carbon Dioxide
6.1 Hydrogen
Hydrogen is the first atom in the universe
Everything else is made from Hydrogen atoms, inside of stars
Hydrogen and Oxygen combine to make Water
Hydrogen gas can be obtained by splitting water back into Hydrogen and Oxygen
If sufficient heat is provided, (like from a flame) then Hydrogen will smash into Oxygen to create Water. This chemical reaction releases a lot of heat and light energy. This heats up all the other hydrogen and oxygen atoms in the area, resulting in a extremely fast chain reaction, a chemical reaction in which hydrogen atoms combine with the oxygen atoms making more water. We hear this as it happens so fast, that the heat released creates a shockwave that we hear as a bang. With the heat and light we see it as an explosion!!
The test for Hydrogen in called the 'pop' test. You put a lit splint (a piece of wood that is on fire) into the gas and it explodes with a "pop".
6.2 Oxygen
Oxygen is an atom that does not like being alone at all. You can find oxygen bound to another oxygen atom. This makes O2. This pairing is known as molecular oxygen, O2.
You can also find oxygen bound to types of atoms, such as one oxygen and two hydrogen atoms, H2O. This compound is
known as water.
You can also find oxygen bound to the outside of old pieces of iron, this compound is known as Iron Oxide, FeO, or rust.
Oxygen is also found in the compound sugar, C6H12O6.
Here you have 6 oxygen atoms, combined with 6 Carbon atoms and 12 Hydrogen atoms.
In the chemical reaction of cellular respiration, 6 pairs of Oxygen, 6O2, combine with the sugar, C6H12O6.
The chemical reaction releases heat energy and chemical potential energy. After the reaction has occurred, and all the atoms have rearranged, you end up with 6CO2 molecules (or compounds, you can choose either word here) and 6 molecules of H2O
FIRE
In the chemical reaction of Fire, Oxygen molecules, O2, are combining with Carbon atoms to make Carbon Dioxide CO2.
There is usually Hydrogen atoms as well, as what is burnt is usually a Hydrocarbon. (Hydro = hydrogen, Carbon = carbon). When Hydrogen combines with Oxygen, you get Hydrogen + Oxygen + Hydrogen, also known as H2O or water.
Examples of hydrocarbons include paper, cardboard, wood, coal, gas, LPG, methane, oil, petrol, diesel, muffins, bread, and toast.... and of course, wood.
Chemical reactions are written with the Reactants first, then an arrow, then the Products
So, the chemical process of Fire can be written as:
Reactants: Hydrocarbon + Oxygen
Products: Carbon dioxide + Water
Hydrocarbon + Oxygen → Carbon dioxide + Water
For the chemical reaction of fire, atoms need to be moving fast, super fast!
In the experiment below a candle is melted, turning the solid wax into liquid wax. The candled is then boiled, as it boils the liquid wax turns into wax gas. A little lid is made to keep the evaporated wax vapor trapped inside the beaker. This makes the gas really hot. Any wax atoms that escape are moving really fast, the hotter it gets the faster those atoms move. As they escape they slam into molecular oxygen in the air, O2.
In the first lift you can see the wax gas but nothing happens. The wax is slamming into the O2. and then bounding off. The lid is put back on. The wax vapor gets even hotter over the next 15 seconds. This means that the wax vapor atoms move even faster, soon they are zooming so fast that when they slam into O2. they bond! This bonding creates CO2. as well as H2O (from the H atoms in the wax).
This collision and chemical reaction releases heat energy that speeds up all the other already fast moving atoms which results in more collisions and chemical reactions and more heat energy, which further speeds up all the atoms, so more collisions, chemical reactions and heat and now light - we have a chain reaction - we have fire!
Fire and combustion are two words for the same chemical reaction: smashing something containing carbon and hydrogen into molecular oxygen in the air, with enough speed and force that they all break their bonds and then regroup into the new compounds of carbon dioxide and water - this process releases heat energy which then speeds up other atoms in the area allowing them to also slam into each other that they also break their bonds and regroup - this is a self-sustaining chain reaction that keeps going as long as there is a fuel left (something with hydrogen and carbon) and as long as there is molecular oxygen, O2., left. This process is so hot that it emits light - we see this light as a fire.
Watch it happen in the "autoignition temperature" clip below.
Note, 'autoignition' happens when atoms in a gas are moving so fast that if they collide with oxygen atoms then they will react and bond. This chemical reaction generates heat energy, which starts the chain reaction causing fire. Heating up anything to its autoignition temperature will result in fire (if its done in the air, in outer space nothing will happen because there is no oxygen.
If you put a glowing splint into an environment of high oxygen concentration, then their will be enough successful collisions for the splint to re-ignite
This is the test for oxygen:
Put a glowing splint into oxygen and it will relight
This is called the: "Glowing Splint Experiment"
Light a splint
Blow it out
Put it into the gas while still glowing
It will reignite if the gas is oxygen
6.3 Carbon Dioxide
Carbon is a atom that has 6 protons, 6 neutrons and 6 electrons. Its electron configuration is 2, 4. Thus, it has 4 Valence electrons.
Having just 4 valence electrons means that Carbon is right in the middle of having a full outer shell. To get its 8 electrons it will usually share with other atoms.
Oxygen is atomic number 8 which means it has an electron configuration of 2,6. If it shares 2 of carbons electrons it will then have a full shell. If another oxygen does the same, then carbon will also have a full outer shell, as 4+2+2=8
This is how Carbon dioxide is formed. It is Carbon sharing its electrons with two Oxygens.
We can find this in the fumes from volcanoes.
The Carbon dioxide fumes go into the atmosphere, making up 0.04% of the air. This carbon dioxide helps to trap heat on the earth, making it warm
The carbon dioxide in the air goes into plants and through the chemical reaction of photosynthesis the carbon becomes part of the plant, whilst the two oxygens leave and become part of the air as O2. This molecular oxygen makes up 21%
When an animal eats the plant, they take in the carbon. The animal breaths in molecular oxygen. Through the chemical reaction of respiration, the O2 bonds onto a Carbon and makes Carbon Dioxide. This CO2 then moves through the animals blood to its lungs, it is then exhaled into the air.
As animals we exhale CO2. If we blow this onto a candle the flame will go out.
As fire uses O2 to make CO2, a build up of CO2 slows and then stops this reaction.
A build up of products will usually slow down a reaction. This is the reason your car has an exhaust pipe, so that the products of internal combustion don't build up and stop the reaction.
This is because the atoms must smash into each other for the reaction to happen. If we are wanting a Carbon to smash into O2 to make CO2 then we need lots of carbon and O2 zooming around so that they can collide. As CO2 builds up it floats around too and can get in the way. The more CO2 there is, the more CO2 that is getting in the way, blocking the O2 and Carbon from bonding and stopping the reaction.
As this continues there is fewer and fewer reactions. Reactions give off heat energy which speeds up the atoms making their collision happen with more force and more frequently. Less reactions due to CO2 getting in the way, means less heat energy being released. So the atoms move slower and they collide less frequently, meaning even less heat, meaning the atoms move even slower, until they get so slow, that when they do collide they no longer bond. And then the reaction stops.
Likewise, putting a cup over a candle flame will cause it to use up all the oxygen (reactant) and build up carbon dioxide (product).
We will see the flame get smaller, as carbon dioxide gets in the way of the reactants. It gets smaller as the atoms start to slow down as less heat is being produced as the number of collisions per second is reducing. It then flickers, more carbon dioxide is getting in the way, less collisions between carbon and oxygen, less heat, slower atoms. Then it goes out as the few atoms that do collide are no longer hitting each other fast enough to bond
Carbon dioxide is heavier than oxygen, this means that you can do some cool experiments with this. If you put candles on different heights, then put a jar or bowl over it then the lowest candles will go out first.
The test for Carbon dioxide is to put a lit splint (a piece of wood that is on fire) into the Carbon dioxide. Because there is too much carbon dioxide in the way, the oxygen wont be able to get to the fire and the fire will go out
Carbon dioxide test: Lit splint into gas, it goes out
Dylan's Chemical Reaction Animation
A former student of mine, Dylan Li, created this animation in Y10.
From your learning this term, have a think about what he is showing through his animation. It's pretty clever :-)
Mr Cowley Lectures
Lecture 1: Structure of an Atom and the Periodic Table - Learning Intentions 1, 2 and 3
Lecture 2: Elements, Molecules, Compounds, Mixtures and Chemical Formula - Learning Intention 4
Lecture 3 - Physical Change, Chemical Change - Learning Intention 5
Lecture 4 - Gases - Hydrogen, Oxygen, Carbon dioxide - Learning Intention 6
Practice Exam Questions
